1. Field of the Art
The present invention relates to a hydraulic control apparatus for controlling a hydraulically-operated power transmitting system used in an automotive vehicle, having a continuously variable transmission (hereinafter referred to as "CVT").
2. Related Art Statement
A power transmitting system for a vehicle including a CVT is provided with an auxiliary transmission for changing a direction of drive of the vehicle and for improvement in drivability.
For example, a power transmitting system is disclosed in Japanese Patent Application Nos. 58-144985 (laid open under Publication No. 60-37455) and 59-12017, which includes a CVT, and an auxiliary or secondary transmission having a frictional coupling device. In such a transmitting system, a common hydraulic line pressure is used to operate the CVT and the frictional coupling device of the auxiliary transmission.
In the case where the auxiliary transmission is coupled to the output side of the CVT, an input torque of the CVT is equal to an output torque of an engine of a vehicle on which the transmitting system is provided. This arrangement is advantageous in terms of constructional simplicity or compactness and durability of the CVT. It is recognized that an input torque of the auxiliary transmission (i.e.,) a torque acting on the frictional coupling is varied with a speed ratio "r" (N.sub.in /N.sub.out, where N.sub.in and N.sub.out represent rotating speeds of input and output shafts, respectively). Accordingly, it is necessary to determine a line pressure of the transmitting system so that the level of the line pressure is high enough to cope with the maximum input torque of the auxiliary transmission at each level of output torque of the engine. However, the operation of the transmitting system with such a relatively high level of line pressure is disadvantageous in term of durability of the frictional coupling assembly of the auxiliary transmission and the CVT.
When the auxiliary transmission of the transmitting system has plural forward-drive shift positions, inadequate control of the line pressure will cause the auxiliary transmission to suffer a shift-up or shift-down shock upon shifting from one position to another, thereby degrading a driving feel or comfort of the automotive vehicle. In the meantime, the line pressure must be held relatively high for avoiding slipping actions of a transmission belt of the CVT. Thus, it has been considered difficult to restrain the shifting shock of the auxiliary transmission while avoiding the slippage of the CVT belt.